1. Field of the Invention
The present invention relates to an apparatus capable of directly detecting fluctuations of switching speed of a logic circuit arrangement in a digital integrated apparatus.
2. Description of the Related Art
As digital integrated circuits have become more reduced in size, a lower power supply voltage has been adopted to alleviate an electric field applied to elements, thus improving the reliability of the elements. Even under a lower power supply voltage, in order to ensure a stable circuit operation, fluctuations in circuit characteristics due to temperature fluctuations, power supply voltage fluctuations, and processing fluctuations must be compensated for.
A prior art apparatus for detecting fluctuations of circuit characteristics includes an N-channel enhancement-type MOS transistor and a resistor in series arranged between a ground voltage terminal and a power supply terminal. In this case, the resistance value of the resistor is so large that a voltage at a connection between the transistor and the resistor is approximately equal to the threshold voltage of the transistor. Therefore, a back gate voltage of a plurality of transistors can be controlled by this voltage, or the size of a plurality of transistors connected in parallel can be quasi-changed by this voltage, thus compensating for the fluctuations of circuit characteristics due to temperature fluctuations, power supply voltage fluctuations and processing fluctuations. This will be explained later in detail.
In the above-described prior art apparatus, however, although the fluctuations of circuit characteristics is detected, fluctuations of switching speed cannot be detected, and thus, it is impossible to compensate for the fluctuations of switching speed.